Nowadays, vehicles equipped with a so-called smart key system have spread through the market in response to requirements of convenience and safety. The smart key system is configured to exchange authentication information by radio between a portable device (smart key) carried by a user and a vehicle opening-and-closing member locking-and-unlocking apparatus when the user of a vehicle approaches or contacts the vehicle opening-and-closing member such as a vehicle door and to automatically lock and unlock the vehicle opening-and-closing member when the authentication information matches (for example, see JP 2002-295094A (Reference 1)).
The smart key system of the related art generally includes (1) an antenna for exchanging authentication information with a portable device carried by a user of a vehicle, (2) a human detection IC that detects contact to a human detecting area provided near a vehicle opening-and-closing member by the user, and (3) an ECU (Electronic Control Unit) that drives and controls the antenna and the human detection IC.
However, with the configuration of the related art as described above, a number of connecting lines are required for connecting components. Therefore, there arises a problem of increases in size and costs of the apparatus. For example, the configuration disclosed in Reference 1 requires at least six harnesses having a length of 5 to 8 m.
As the related art for solving the above-described problem, there is proposed an apparatus which is reduced in size and costs by integrating the antenna and the human detection IC in one vehicle-mounted apparatus and superimposing a direct current power source voltage for the human detection IC, a human detecting signal and an antenna drive signal on a single connecting line and transmitting the aforesaid superimposed signals through the corresponding connecting line (for example, see Japanese Patent No. 5589870 (Reference 2)).
In Reference 2, a plurality of connecting lines that connect the ECU and the vehicle-mounted apparatus are combined into two connecting lines; a first line and a second line, and the direct current power source voltage and the antenna drive signal are switchably output from the two connecting lines to avoid an increase of the maximum value of the signals superimposed on the first line. An antenna resonance voltage is used as a power source for the human detection IC so as to maintain a human detection function also when the antenna is driven and thus the direct current power source voltage is not supplied.
However, the related art has the following problems.
For example, in Reference 2, when the antenna is driven, the antenna resonance voltage is used as power supplied to the human detection IC instead of the antenna drive signal output from the ECU. However, the antenna resonance voltage, being a value obtained by multiplying the antenna drive signal by a Q value of the antenna, is a value significantly higher than a rating of the human detection IC.
Therefore, with the configuration disclosed in Reference 2, the Q value of the antenna needs to be lowered so as to comply with the rating of the human detection IC in order to prevent the antenna resonance voltage from exceeding the rating of the human detection IC. Consequently, an output from the antenna becomes insufficient, and thus arrangement of the antenna needs to be devised in order to compensate the insufficiency of the output, which causes a problem of an increase in size of the antenna.
In contrast, an impedance Z0=jωL+1/jωC of a series resonance circuit of the antenna becomes almost zero at a resonance frequency ω=1/√(LC) that the antenna drive signal outputs. Therefore, a current generated by the antenna drive signal flows toward an antenna (20) instead of a human detection IC (24) when the antenna resonates, and thus cannot be used as power for the human detection IC (24).
Even though the rating of the human detection IC is increased so as to comply with the antenna resonance voltage instead of reducing the antenna resonance voltage, the human detection IC and peripheral circuits thereof need to have high ratings. Therefore, the problem of increases in size and costs of the apparatus still remains.